Heat recovery regenerators have been used in many industries for many decades to recover usable heat from high temperature exhaust streams mainly for the purpose of reducing fuel consumption. One familiar application for heat recovery using regenerators is in air fired glass melting furnaces. Some typical types of regenerators use stacks of refractory checkers which are supported by an arch and enclosed in a refractory enclosure having an opening at both ends for the high temperature gas formed in the furnace to pass through. The hot gas enters the regenerator one end (usually located at the top) and exits from the other end (usually located at the bottom) at a lower temperature as part of the heat content of the exhaust gas is transferred to and stored in the checkers. After the checkers are sufficiently heated the regenerator undergoes so called reversal in which hot gas flow through the regenerator is stopped and combustion air (to be combusted with fuel in the furnace) enters the regenerator bottom and then exits at the top of the regenerator, having been preheated in the regenerator, and is then fed into the furnace for combustion with fuel in the furnace. Typical time between the reversals is about 20 to 30 minutes for regenerators used for glass melting furnaces. The temperature of the preheated combustion air may reach 1250 to 1350 C depending on several factors such as age, maintenance, and the design of the regenerator. Rotary regenerators are another type of heat recovery devices, in which the hot and the cold gas streams flow counter-currently in their own ducts toward segmented heat-storage mediums installed in a rotating device. Heat exchange is accomplished when the heat storage mediums are rotated alternatively between the hot and the cold fluids. In this case the heat storage mediums can be materials made of for example metal ribbons, wire screens, refractory honeycombs, or ceramic balls. This type of regenerators is used in the steel industry and in utility boilers for air preheating.
One difficulty associated with the operation of the heat recovery regenerators is that when the high temperature flue gas stream comprises condensable alkalis, sulfate and/or carbonate salts may form as the hot exhaust gas is being cooled in the regenerator. For example, hot flue gases from a glass furnace may have vapor species containing sodium or potassium such as NaOH, KOH and NaBO2 which originate from molten glassmelt, glass making batch materials, and also sulfur dioxide which is a common component from fossil fuel combustion. During cooling of flue gas, at a temperature in the range of approximately 800 to 1100 C, salts such as Na2SO4 and K2SO4 may form from gaseous alkali vapors, sulfur dioxide and oxygen and condense from the flue gases in liquid form, and other fine solid materials present in the flue gases such as batch carryover may adhere to the checker surfaces as liquid or solid deposits. If these deposits are not removed or cleaned over a long period of time, the gas passages in the checker passageways within the regenerator may be increasingly blocked which can eventually result in a partially or substantially plugged regenerator.
A plugged regenerator presents serious challenges to furnace operators in maintaining production rate and in keeping regular maintenance schedules. Narrowed checker passages reduce air flow capacity passing through a regenerator because the pressure side of the blower for moving the combustion air has a design limit. In addition, the deposits on checker surfaces hinder heat transfers both from the hot gas to the checker and from the checker to the air which is to be preheated. Air preheat temperature from a plugged regenerator can be lower than normal. Lower available air flow capacity may force operators to reduce glass pull rate because fuel has to be reduced to match the available air flows, therefore suffer economic losses. Furthermore, lower air preheat temperature results in lower flame temperature in the furnace, which in some situations may induce operational issues such as the quality of glass formed in the furnace. Needless to say a plugged regenerator also reduces heat recovery from the hot gas, and thus specific energy consumption (i.e., energy usage per ton of glass or other product melted or formed in the furnace) can increase which reduces the operator's profit margin.
A plugged regenerator used in glassmelting furnaces is typically cleaned periodically, typically once or twice a year. A common method is “thermal cleaning”, in which the temperature of the checker pack of the plugged regenerator is increased to a sufficiently high level to melt down the deposits. Melted deposits are collected at the bottom of the regenerator chamber and removed. An additional heat generating source such as an oxy-fuel burner is often used in the bottom space below the checker pack to increase the checker pack temperature. It is a delicate and slow operation conducted over a few days as overheating can potentially damage or collapse the checker pack. During such cleaning operation the plugged regenerator is partially cleaned and the thermal efficiency is partially restored. However, the cleaning operation interrupts the normal operation of the glass furnace and results in a production loss. There is a need to develop an improved heat recovery process that prevents the plugging of regenerator passages by deposits build up.